The invention relates to the field of location algorithms for remote devices. More particularly, the invention relates to an algorithm system for determining the position of an electronic pointing device.
Digitizing pen and whiteboard systems are used for a variety of electronic applications. These systems typically include a whiteboard, a position indicating pen, and associated electronics for determining the interaction between the whiteboard and the position indicating pen. A digital data signal is typically derived to represent the relative position of the position indicating pen and the whiteboard.
When a signal, such as ultrasound, is used as a location signal for a remote device, it is often difficult to determine the location of the device accurately, since it is difficult to determine where upon each of sequential long wavepulses to measure, as a determination of the time of arrival to external receivers.
M. Stefik and C Heater, Ultrasound Position Input Device, U.S. Pat. No. 4,814,552 (Mar. 21, 1989) discloses an xe2x80x9cinput device, or stylus, for entering hand drawn forms into a computer using a writing instrument, a pressure switch for determining whether the instrument is in contact with the writing surface, an acoustic transmitter for triangulating the position of the stylus on the surface, and a wireless transmitter for transmitting data and timing information to the computer. In operation, the stylus transmits an infrared signal which the system receives immediately, and an ultrasound pulse which two microphones receive after a delay which is a function of the speed of sound and the distance of the stylus from the microphonexe2x80x9d. While Stefik et al. discloses an algorithm to analyze the incoming ultrasound signals to locate the stylus, the algorithm computes radii to each of the two microphones using information from only a single sonic pulse sample, translates the two radii into a calculated X,Y location, and then filters the calculated X,Y values, removing them from the described path if they vary from a specified limit, or range.
B. Edwards, Ultrasound Position Locating Method and Apparatus Therefor, U.S. Pat. No. 5,142,506 (Aug. 25, 1992) discloses a xe2x80x9cpositional locating method and apparatus for measuring distances by accurately determining the transit time of ultrasonic wave bursts between two or more pointsxe2x80x9d. xe2x80x9cTimer clocks are started when each of the bursts is triggered to be emitted from a transmission point, and are stopped when a highly defined point in the burst is received at a corresponding receiving point. The highly defined point is determined by first analyzing the burst to identify a particular cycle within the burst. The particular cycle is then analyzed to detect the specific point within the cyclexe2x80x9d.
Analog systems, such as described by Edwards, are inherently limited to xe2x80x9con the flyxe2x80x9d comparison between a current signal burst and a small amount of amplitude information from a single prior signal. Since analog systems do not store the entire prior signal bursts in memory, they are limited to the comparison of a small number of features on the last prior signal.
While Edwards typically uses multiple receivers to locate a transmitter using ordinary trigonometric calculations, the analog system is limited to the comparison of amplitude between a small number of measured peaks on successive cycles within xe2x80x9cburstsxe2x80x9d of the received ultrasonic waveform. Common variations of the waveform, typically due to ordinary use of a transmitter, either from the orientation of the transmitter to the receivers, the speed at which the transmitter is moved between different regions of a writing surface, the signal strength of the transmitted signal, noise, or reflections can result in erroneous results. Reliance on the amplitude of a specific cycle within a pulse waveform can lead to errors of one or more cycles, resulting in position detection errors of several centimeters. Errors in such an analog system commonly result either in an inaccurate determined location for the transmitter, or in a determined location point which is required to be xe2x80x9cthrown outxe2x80x9d from the described path of the movable transmitter. As well, the analog system used inherently limits the type of comparison between the amplitude of selected cycle peaks within signal xe2x80x9cburstsxe2x80x9d within a prior output signal and a current output signal, thus preventing the analog system to being easily adaptable to hardware embodiments or improved waveform comparison techniques.
I. Gilchrist, Acoustic Mouse System, U.S. Pat. No. 5,144,594 Sep. 3, 1992) discloses an acoustic mouse system, which xe2x80x9ccontrols indications on an X-Y surface of the face of a display. The system comprises at least three acoustic receivers in an x-y plane, and a hand movable acoustic transmitter that is movable both parallel to the x-y plane and in a z direction perpendicular to the x-y plane. The transmitter generates periodic acoustic oscillations in the direction of the support and its receivers. Detection circuitry, responsive to the signals from the acoustic receivers, provides signals indicative of the absolute position of the acoustic transmitter in the x-y plane. A processor is responsive to the signals from the detection circuitry to provide absolute position signals to the display, whereby the display responds by moving an indication to a corresponding position on the X-Y surface of the display face. The detector circuitry is further enabled to provide z position signals to the display, whereby the display may modify a display function in accordance with the z position signalsxe2x80x9d. While Gilchrist discloses a generic, periodic acoustic wavelength position indicating system, Gilchrist fails to disclose a useful algorithm by which the position of the movable acoustic transmitter is determined by the detection circuitry, either by the direction of arrival or by the time of arrival of a transmitter signal. Furthermore, the system apparently requires a minimum of three acoustic receiver locations to properly locate the movable acoustic transmitter. Gilchrist also fails to disclose waveform analysis techniques which can be used to provide sufficient accuracy in the determination of the movable acoustic transmitter.
The disclosed prior art systems and methodologies thus provide basic transmitter pen and whiteboard positioning systems for determining the spatial relationship between a pen and a writing area, but fail to provide an accurate means for determining the position of the tip of the pen. The development of such a transmitter pen positioning system would constitute a major technological advance. Furthermore, the development of such a transmitter pen positioning system which also provides a means for communicating supplementary information between a transmitter pen and external receivers would constitute a further major technological advance.
A transmitter pen location system is provided, in which a pen is adapted to send a repeated output signal to external receivers at two or more receiver locations, wherein the location of the pointing tip of the pen is determined in relation to the writing surface of a whiteboard. In one embodiment an output element, preferably an ultrasonic transducer, transmits an output signal from the transmitter pen to two or more external receivers at two or more receiver locations. The direction of arrival of the output signal to each of the receiver locations is determined, and the position of the pointing tip of the transmitter pen is then determined, as the intersection of direction vectors from the pen to the external receivers. In an alternate embodiment, the transducer transmits an output signal from the transmitter pen to three or more external receivers, wherein the received signal is processed to determine the time of arrival to each of the receivers, and the location of the pointing tip of the pen is determined as the calculated distance between the transducer and each of the receivers. In another alternate embodiment having a dual signal transmitter pen, a third receiver located at a receiver location is used to determine the time of arrival of a secondary output signal, to determine the distance between the dual signal transmitter pen and the receiver location, while two or more receivers at the receiver location are used to determine the direction of arrival of the primary output signal. Alternative embodiments allow the transmission of supplementary information from the transmitter pen to the receivers, using waveshaping of the output signal.